A blade for a wind turbine, also sometimes referred to as a rotor blade, is usually manufactured using a technique such as the closed-mould casting technique in which the entire blade can be moulded in one piece. Such a mould is usually made in two parts, with an upper half and a lower half. Glass fibre matting can be used to build up component layers in a suitably shaped mould, and the layers of matting are bonded with a resin and cured in the mould in a vacuum-assisted resin transfer moulding (VARTM) process to give a fibre-reinforced polymer or glass-reinforced plastic, generally referred to simply as ‘fibreglass’. Stacked layers of resin-coated matting are generally referred to as a ‘layup’ or ‘composite layup’. Additional resin can be pumped into the closed mould by means of a vacuum, so that the component layers are caused to press against the inside mould surface, thus assuming the desired end shape. The finished blade can then be processed (for example sanded and painted) and machined in preparation for fixing the blade to another component (for example holes may be drilled to receive bolts).
For a blade that is to have a clearly defined pressure side and a suction side, each half of the mould is shaped specifically so that the finished or cast blade has the desired form. Evidently, such an asymmetrical blade must be mounted to a hub or a pitch bearing at a precise angle, otherwise the performance of the wind turbine might be adversely affected. Therefore, before drilling holes for bolts that are used to attach a connecting part—for example a flange—to the base of the blade, or before milling the bottom edge of the blade at an angle for correct pitch, the correct orientation of the blade must be determined. In prior art manufacturing techniques, the blade (which can easily be 40 m-50 m or more in length) is supported on a frame, and the relative location of an unambiguous reference point, for example, the point at the outer edge of the blade at its widest part, is determined relative to the essentially circular base of the blade. Because of its round form, the base of the blade is generally referred to as the ring section or root portion. The large dimensions of the blade require a correspondingly long tool or device that can span the length between the reference point and the root portion. Determining a reference point in this way can be time-consuming and can add to the overall cost of the blade. Furthermore, such a technique can be error-prone, so that a machining step may not be sufficiently precisely carried out. Bolt holes that are not exactly drilled at exactly the right points, or a coning angle that is slightly offset, may be impossible to correct and may ultimately result in damage to load-bearing parts such as the pitch bearing.